The present invention relates generally to printers, and more particularly to a method for determining printhead misalignment of a printer.
Printers include inkjet printers having one or more printheads used to print on print media. An inkjet printhead typically includes a vertical array of inkjet nozzles. In some designs, the vertical array is a single line array aligned perpendicular to the printhead scan direction or aligned slightly tilted from perpendicular when the nozzles in the line array are fired with a time delay as is known to those skilled in the art. In other designs, the vertical array includes two or more vertical line segments horizontally spaced apart with the nozzles in one vertical line segment fired with a time delay relative to the nozzles in another vertical line segment as can be appreciated by the artisan. In still other designs, the vertical array includes two or more horizontally spaced-apart vertical lines or line segments, wherein a nozzle of one vertical line or line segment is positioned vertically between two adjacent nozzles of another vertical line or line segment. The term xe2x80x9cprintheadxe2x80x9d means a group of pixel printing elements capable of causing any possible character or symbol (including a single or multi-pixel character or symbol) of a single color to be printed on the print media. The term xe2x80x9cprintheadxe2x80x9d also includes the terms xe2x80x9cpenxe2x80x9d and xe2x80x9ccartridgexe2x80x9d. A typical color inkjet printer has a black printhead and three color printheads (such as a cyan printhead, a yellow printhead, and a magenta printhead). In some designs, the three color printheads are three groups of nozzles on a single printhead block mounted to the printhead carriage. Printers having horizontally spaced-apart redundant printheads are known.
Print quality depends on the skew alignment of each printhead with respect to the printhead scan direction, on the bi-directional alignment of each printhead in the forward printhead scan direction relative to the reverse printhead scan direction, and on the horizontal and vertical alignments of one printhead relative to another printhead. A conventional method of printhead alignment includes printing an alignment pattern (having spaced-apart images) on the print media, passing a printhead-carriage-mounted optical sensor along the printhead scan direction over the alignment pattern to detect the alignment pattern, using a counter-timer to measure the time it takes the optical sensor to reach the leading and/or trailing edges of the images of the alignment pattern, calculating the positions of the images from the measured times of the counter timer, and determining the printhead misalignments from the calculated image positions. Another conventional method uses the printhead carriage encoder to determine the position of the images detected by a printhead-carriage-mounted optical sensor. Some of these methods use computationally-intensive algorithms requiring large memory space.
What is needed is an improved method for determining a printhead misalignment of a printer.
A first method of the invention is for determining a printhead misalignment of a printer and includes steps a) through e). Step a) includes printing a printhead alignment test pattern including spaced-apart images at least partially aligned substantially along a printhead scan axis. Step b) includes moving a sensor along the printhead scan axis at a known speed over the plurality of images. Step c) includes obtaining sampled data points from the sensor at a known sampling rate. Step d) includes determining the locations along the printhead scan axis of the edges of the images using the sampled data points, the known speed of the sensor, and the known sampling rate. Step e) includes calculating the printhead misalignment from the determined locations of the edges of the images.
A second method of the invention is for determining a printhead misalignment of an inkjet printer and includes steps a) through e). Step a) includes printing a printhead alignment test pattern including spaced-apart block images at least partially aligned substantially along a printhead scan axis. Step b) includes moving a printhead-carriage-mounted optical sensor along the printhead scan axis at a known printhead carriage speed over the block images. Step c) includes obtaining sampled data points from the optical sensor at a known sampling rate. Step d) includes determining the locations along the printhead scan axis of the edges of the block images using the sampled data points, the known printhead carriage speed of the optical sensor, and the known sampling rate. Step e) includes calculating the printhead misalignment from the determined locations of the edges of the block images.
A third method of the invention is for determining a printhead misalignment of a printer and includes steps a) through e). Step a) includes printing a printhead alignment test pattern including spaced-apart images at least partially aligned substantially along a printhead scan axis. Step b) includes moving a sensor along the printhead scan axis at a known speed over the images. Step c) includes obtaining sampled data points from the sensor at a known sampling rate, wherein the sampled data points are obtained as digitized data points from an analog-to-digital converter whose input is operatively connected to the output of the optical sensor. Step d) includes determining the locations along the printhead scan axis of the edges of the images using the sampled data points, the known speed of the sensor, and the known sampling rate, wherein the digitized data points of the odd-numbered images are compared against a first threshold value to determine the locations of the edges of the odd-numbered images, and wherein the digitized data points of the even-numbered images are compared against a second threshold value to determine the locations of the edges of the even-numbered images. Step e) includes calculating the printhead misalignment from the determined locations of the edges of the images.
A fourth method of the invention is for determining a printhead misalignment of a printer and includes steps a) through e). Step a) includes printing a printhead alignment test pattern including spaced-apart images at least partially aligned substantially along a printhead scan axis. Step b) includes moving a sensor along the printhead scan axis at a known speed over the images. Step c) includes obtaining sampled data points from the sensor at a known sampling rate, wherein the sampled data points are obtained as digital data points from a bi-stable comparator whose input is operatively connected to the output of the optical sensor, and wherein the bi-stable comparator compares the optical sensor output to a single threshold value to set the state of the digital data point output of the bi-stable comparator. Step d) includes determining the locations along the printhead scan axis of the edges of the images using the sample numbers which correspond to changes of state of the digital data points, the known speed of the sensor, and the known sampling rate. Step e) includes calculating the printhead misalignment from the determined locations of the edges of the images.
Several benefits and advantages are derived from one or more of the four methods of the invention. By obtaining sampled data points from the sensor, the positions of the edges of the printed images of the printhead alignment test pattern can be calculated from the known sampling rate and the known sensor speed, and printhead misalignment can be calculated from the determined edge locations. This avoids having to use the printhead carriage encoder or a clock to determine edge locations as is done in conventional methods for determining printhead misalignment of a printer. This also avoids the use of computationally-intensive algorithms.